So what did exist before the Big Bang? This question would normally belong in the realms of deep philosophical thinking; the laws of physics have no right to probe beyond the Big Bang barrier. There can be no understanding of what was there before. We have no experience, no observational capability and no way of travelling back through it (we can’t even calculate it), so how can physicists even begin to think they can answer this question? Well, a new study of Loop Quantum Gravity (LQG) is challenging this view, perhaps there is a way of looking into the pre-Big Bang “universe”. And the conclusion? The Big Bang was more of a “Big Bounce”, and the pre-bounce universe had the same physics as our universe… just backwards… Confused? I am…
LQG is a tough theory to put into words, but it basically addresses the problems associated with the incompatibilities behind quantum theory and general relativity, two crucial theories that characterize our universe. If these two theories are not compatible with each other, the search for the “Theory Of Everything” will be hindered, disallowing gravity to merge with the “Grand Unified Theory” (a.k.a. the electronuclear force). LQG quantizes gravity, thereby providing a possible explanation for gravity and a possible key to unlocking the Theory Of Everything. However, from the outset, LQG has many critics as there is little direct or indirect evidence backing up the theory.
Regardless, much work is being done into this area of research. The primary consequence to come from LQG is that it predicts that the Big Bang which occurred 13.7 billion years ago was actually a “Big Bounce”; our universe is therefore the product of a contracting universe before the Big Bang. The previous universe (or our universe “twin”) contracted to a single point (which could be interpreted as a “Big Crunch”) and then rebounded in a Big Bounce to produce the Big Bang as we’ve learned to accept as the birth of the universe as we know it. But until now, although the pre-bounce universe has been predicted, its characteristics could not be known. No information about the pre-bounce universe could be observed in today’s universe, the Big Bounce causes a “cosmic amnesia”, destroying all information of the previous universe.
Now, physicists Alejandro Corichi from Universidad Nacional AutÃ³noma de MÃ©xico and Parampreet Singh from the Perimeter Institute for Theoretical Physics in Ontario are working on a simplified Loop Quantum Gravity (sLQG) theory where they approximate the value of the “quantum constraint”, a key equation in the LQG theory. What happens next is a little surprising. From their calculations, it would appear that a universe, identical to our own, with identical mechanics, existed before the Big Bounce.
“…the twin universe will have the same laws of physics and, in particular, the same notion of time as in ours. The laws of physics will not change because the evolution is always unitary, which is the nicest way a quantum system can evolve. In our analogy, it will look identical to its twin when seen from afar; one could not distinguish them.” – Parampreet Singh
We are not talking about an alternate dimension; we are talking about an identical universe with the same space-time and quantum characteristics as our own. If we look at our universe now (13.7 billion years post-bounce), it would be identical to the universe 13.7 billion years before the Big Bounce. The only difference being the direction of time would be opposite; the pre-bounce universe would be reversed.
“In the universe before the bounce, all the general features will be the same. It will follow the same dynamical equations, the Einstein’s equations when the universe is large. Our model predicts that this happens when the universe becomes of the order 100 times larger than the Planck size. Further, the matter content will be the same, and it will have the same evolution. Since the pre-bounce universe is contracting, it will look as if we were looking at ours backward in time.” – Parampreet Singh
Analysing what happened before the Big Bang is only part of the story. By making this approximation of a key LQG equation, Singh and Corichi are working on models where galaxies and other physical structures leave an imprint in the pre-bounce universe to influence the post-bounce universe. Would these structures be distributed in similar ways? Will the structures in one universe be similar or identical to structures in the other universe? There may also be an opportunity to look into the future of this universe and predict whether the conditions are right for another Big Bounce (once can imagine repeated bounces, producing a cycle of universes).
For now, this research is highly theoretical and any observational evidence will remain sparse for the time being. Although this is the case, it does begin to probe the big question and may push physics a bit closer toward describing what existed before the Big Bang…